This invention relates to an anti-lock control method for preventing a wheel lock when braking a running vehicle.
Generally, in an anti-lock control apparatus for a vehicle, for the purpose of securing the steerability and the running stability of the vehicle and of reducing the braking distance, a control mode of brake hydraulic pressure is determined in accordance with an electric signal representative of a wheel speed detected by a wheel speed sensor, and a hold valve in the form of a normally-open solenoid valve and a decay valve in the form of a normally-closed solenoid valve are opened and closed, so that the brake hydraulic pressure is controlled by a control unit, including a microcomputer, to be increased (pressurized), held and decreased.
In such an anti-lock control, each wheel speed to be controlled is set as a system speed Vs in a respective one of a plurality of hydraulic pressure systems.
FIG. 5 is an illustration of such anti-lock control, showing variations in the system speed Vs, the wheel acceleration and deceleration dVs/dt and the brake hydraulic pressure Pw as well as a hold signal HS for opening and closing the hold valve and a decay signal DS for opening and closing the decay valve.
When the brake is in an inoperative condition during the running of the vehicle, the brake hydraulic pressure Pw is not increased, and since the hold signal HS and the decay signal DS are both in the OFF state, the hold valve is in an open condition, and the decay valve is in a closed condition. With the braking operation, the brake hydraulic pressure Pw is pressurized from time t0 to abruptly increase (normal mode), so that the system speed Vs decreases. There is set a reference wheel speed Vr which follows the system speed Vs in such a manner that the reference wheel speed Vr is kept lower by a constant speed .DELTA.V than the system speed Vs. When the deceleration (negative acceleration) dVs/dt of the system speed Vs reaches a predetermined threshold value (for example, -1G) at time t1, the reference speed Vr is set to linearly decrease from this time t1 at a gradient .theta. of deceleration of -1G. Then, at time t2 when the deceleration dVs/dt of the system speed Vs reaches a threshold value -Gmax (for example, -2G) representing a predetermined maximum deceleration, the anti-lock control is started, and the hold signal HS is turned to the ON state to close the hold valve, thereby holding the brake hydraulic pressure Pw.
As a result of holding the brake hydraulic pressure Pw, the system speed Vs is further decreased, and becomes lower than the reference speed Vr at time t3. At this time t3, the decay signal DS is turned to the ON state to open the decay valve, thereby starting the decrease of the brake hydraulic pressure Pw. Due to this pressure decrease, the system speed Vs begins to increase from time t4 of its low peak. At this time t4 of the low peak, the decay signal DS is turned to the OFF state to close the decay valve, thereby finishing the decrease of the brake hydraulic pressure Pw to hold the brake hydraulic pressure Pw. The system speed Vs reaches the high peak at time t7, and the increase of the brake hydraulic pressure Pw is again started from this time t7. With respect to this pressure increase, the increase and hold of the brake hydraulic pressure Pw are alternately effected repeatedly by switching the hold signal HS between the ON state and the OFF state at relatively small time intervals, thereby gently increasing the brake hydraulic pressure Pw to decrease the system speed Vs, and the pressure decrease mode is again started at time t8 (corresponding to time t3).
On the other hand, an estimated vehicle speed Vv is set as an approximate value of the vehicle speed. This estimated vehicle speed Vv is obtained by selecting the highest wheel speed VwH out of the four wheel speeds (four-wheel select-high) and by restricting a following limit relative to this highest wheel speed (four-wheel select-high speed) VwH, for example, to a range of .+-.1G.
Time t5 when the system speed Vs has recovered from a low peak speed Vl to a speed Vb(=Vl+0.15Y) which is higher than the low peak speed Vl by an amount corresponding to 15% of a speed difference Y between a system speed Va at time t3 of the start of the pressure decrease and the low peak speed Vl, as well as time t6 when the system speed Vs has recovered from the low peak speed Vl to a speed Vc(=Vl+0.8Y) which is higher than the low peak speed Vl by an amount corresponding to 80% of the above speed difference Y, is detected. A first pressure increase period Tx starting from time t7 is determined by a road surface friction coefficient .mu. based on the calculation of an average acceleration (Vc-Vb)/.DELTA.T obtained during a period .DELTA.T between time t5 and time t6. Subsequent pressure hold period or pressure increase period is determined based on the deceleration dVs/dt detected immediately before such pressure hold or pressure increase.
By the combination of the above increase, hold and decrease of the brake hydraulic pressure Pw, the system speed Vs is controlled so as to decrease the vehicle speed without locking the wheels.
Generally, when such an anti-lock control method is applied to the vehicle, there has widely been used a three-channel anti-lock control method in which the speeds of left and right front wheels represent first and second system speeds, respectively, and a lower speed out of the speeds of left and right rear wheels is selected (select-low) and is used as a third system speed, and the brake hydraulic pressures with respect to these three system speeds are controlled independently of one another, thus effecting so called front wheel independent-and-rear wheel select-low anti-lock control method.
As is clear from the foregoing, in the conventional anti-lock control method, in order to increase the S/N ratio in view of road surface noises or the like, the deceleration threshold value at which the reference speed Vr is changed to a constant deceleration gradient is set to a value (for example, -1G) greater than a deceleration value of the vehicle speed produced by an ordinary deceleration. It is detected that the deceleration of the system speed Vs reaches the above predetermined threshold value -1G, and based on this detection, the reference speed Vr is decreased from time t1 at a deceleration gradient .theta., and at time t3 when the system speed Vs becomes lower than the reference speed Vr, the decrease of the brake hydraulic pressure Pw is started. Therefore, when with a gentle braking operation, the system speed Vs is decreasing at such deceleration -dVs/dt (for example, -0.7G) that the system speed Vs does not reach the predetermined threshold value -1G, the reference speed Vr only follows the system speed Vs with the speed difference .DELTA.V, and hence does not intersect the system speed Vs. Therefore, the deceleration continues apart from the vehicle speed, with the decrease point of the brake hydraulic pressure Pw not detected, and as a result there is a possibility that a premature wheel lock may occur on a low-.mu. road surface.